This invention relates generally to contact switch assemblies and methods for their manufacture, and more particularly to a contact switch assembly for use in a keyboard having one or more keys.
Presently available contact switch assemblies for use in keyboards generally include a printed circuit board having a plurality of circuits, each of which terminates with first and second contacts located proximately to each other. These printed circuit boards may have the circuit conductors provided only on the top or front surface of the board or, alternatively, on both the top and bottom surfaces in which case the circuit conductors are printed on the bottom or back board surface with the contacts extending through the board to the front surface through a plated hole or equivalent opening in the board. A resilient contact plate, such as a snap-acting dome, cooperates with each pair of contacts to perform the switching function in a known manner. The invention described below can be utilized with either type of printed circuit board and is not limited to use with the particular type illustrated in the drawings.
One of the difficulties involved in the above described contact switch assemblies has been the manner in which the snap-acting domes are fixed in position on the printed circuit board. Each dome must be precisely located and fixed in position so that at least a portion of its periphery electrically communicates with one of a pair of contacts so that, upon deflection, an inner portion of the dome will move into communication with the other contact thereby closing that particular circuit. A technique in current use for fixedly locating the domes comprises providing a thin sheet of polyester material or the like having a pattern of openings punched in it which precisely corresponds to the desired pattern of the domes on the circuit board. This type of sheet which may be called a "dome cage," an example of which is shown in FIGS. 4-6 of U.S. Pat. No. 3,749,859 granted to Webb et al. on July 31, 1973, has a pressure-sensitive adhesive layer on each of its surfaces so that subsequent to the punching operation, the cage is adhesively connected to the printed circuit board. The domes are then subsequently loaded into the individual locations defined by the openings in the cage and are thereby correctly positioned relative to the contact pairs. A cover sheet is then provided over the outwardly facing adhesive surface of the dome cage thereby completing the assembly which comprises the circuit board, the cage and the domes, whereupon the covered assembly may be heated and pressed to insure good adhesion between the cage and the printed circuit board. A precise second punching operation may be used subsequent to positioning the cover sheet to form mounting apertures and/or switch holes.
These dome cages, however, are not entirely satisfactory. The cost of the cage material, adhesives, and the labor involved in providing the adhesive on the cage material is relatively expensive. The quality of the adhesive and cage material has been found to vary. Due to this inconsistency in quality of the adhesive and cage material, the punching process not infrequently causes the dome cage to crack or its surfaces to become contaminated with particles of adhesive and dome cage material. Relatively expensive and time-consuming cleaning and inspection steps are necessary in order to insure that these extraneous particles are kept to a minimum. Further, the openings formed in the dome cage must be precisely located with exact tolerances to assure proper electrical contact. This requires extensive, costly tooling equipment. If the temperatures occurring during the final heating step are too high, adhesive might flow into the dome area often resulting in malfunctioning. Additionally, if temperatures were too low, the final punching step frequently caused the circuit board to crack. Thus, a precisely controlled oven is required which adds yet further expense and additional time to the manufacturing process.